The government's air accident investigation branch is expected to ask for details of why a plane landed at a Northern Ireland military airfield.

Investigators will focus on why the pilots failed to identify the correct airfield despite clear weather and warnings about the Ballykelly airstrip.

Eirjet said the pilot is "not flying" until the incident is thoroughly investigated by aviation authorities.

The Liverpool-Londonderry flight was carrying 39 passengers at the time.

Eirjet, which was operating the flight on behalf of Ryanair on Wednesday, apologised but said the safety of the passengers was not compromised.

Ballykelly and City of Derry runways are aligned in the same direction.

Pilots' manuals outline the possibility of confusion in flight manuals over the airports, which are five miles apart.

Eirjet said the incident "involved the aircraft landing at a runway exactly in line" with the airport's runway.

Human beings are fallible - from simple things like putting teabags in a milk jug to the other end of the spectrum of landing at the wrong runway

Captain Mervyn GranshawBritish Air Line Pilots' Association

On Thursday, the manager of City of Derry Airport said it was too early to conclude what the reason was for the incident.

Seamus Devine said it may have been a case of the pilot relying on what he could see, rather than on his instruments.

"An error took place somewhere and an aircraft ended up at an airfield it shouldn't have gone into," said Mr Devine, who was a pilot for 20 years.

He added: "A pilot looks at the window, sees a runway, the human instinct - the visual cortex - takes over and you see a runway immediately below you, distrust your instruments, and go around and land on that runway."

'Plug those gaps'

Captain Mervyn Granshaw, chairman of the British Air Line Pilots' Association, said there were several reasons why such an incident could occur.

"Clearly, it is a very worrying issue," he said.

"Human beings are fallible - from simple things like putting teabags in a milk jug to the other end of the spectrum of landing at the wrong runway.

"We know this in aviation, but we try to put in place processes, checks, balances to make sure we plug all those gaps."

In over seven years of Ryanair flights into City of Derry airport, and over 20 years of Ryanair-operated flights, such a mistake has never occurred before

Ryanair

Ryanair said in a statement it was due to an "error by the Eirjet pilot who mistakenly believed he was on a visual approach to City of Derry airport".

The passengers were taken by coach from Ballykelly to the airport.

The airstrip at Ballykelly is primarily used by Army helicopters and light aircraft.

It was built for huge military planes, making it one of the longest on the island.

Eirjet said it would begin a full investigation, working in consultation with the Department of Transport, the Irish Aviation Authority and the Civil Aviation Authority.

The airline said it placed "paramount importance on passenger safety".

Ryanair said it would be carrying out its own investigation into the incident.

In a statement, the company said: "In over seven years of Ryanair flights into City of Derry airport, and over 20 years of Ryanair-operated flights, such a mistake has never occurred before."